1. Field of the Invention
This invention relates in general to equipment for transferring fluids. In particular, the invention relates to a fluid swivel joint and a swivel stack assembly adapted for transferring fluids between tankers, storage vessels and the like and one or more conduits beneath the ocean surface. The fluid of the swivel may be product such as hydrocarbons to be transferred from the seabed to a vessel or may be water or gas to be transferred from the vessel to the seabed for well stimulation.
Still more particularly, the invention relates to a sealing arrangement for a fluid swivel joint which uses the pressure of the fluid flowing through the joint to substantially prevent seal glands, and extrusion gaps in which dynamic seals are placed, from enlarging as a function of high pressure of the fluid commonly encountered on offshore loading terminals for oil and gas tankers. The invention also relates to an arrangement by which a spool is coupled between a swivel joint and a swivel stack base. The invention also concerns a procedure for disassembling and reassembling a swivel so that seals of individual swivel assemblies in a swivel stack can be replaced without the need of an overhead crane and without removing an assembly from the stack.
2. Description of the Prior Art
The offshore search for oil and gas has greatly expanded in recent years and progressed into deep rough waters such as the North Sea. To facilitate production of oil and gas from remotely located offshore fields, complex mooring systems for offshore loading terminals which serve as centralized production sites for the entire field have been developed. Flexible fluid lines called risers extend from a subsea location to the mooring site to permit the transfer of fluids between a moored vessel and a subsea location. For example, certain fluid lines may be used to convey oil and gas into the floating vessel while other fluid lines may be used to inject liquids or gases back from the vessel into subsea wells for purpose of control, well stimulation or storage.
Floating vessels can be moored to a single point mooring system, which permits the vessel to weathervane and rotate 360xc2x0 about a single mooring point. To permit the vessel to rotate and move freely without causing twisting or entanglement of the various risers to which the vessel is attached, it is necessary to provide a swivel mechanism to connect the fluid lines to the mooring site. Furthermore, since a plurality of risers are involved, it is necessary that swivels be stacked in order to have the capability of accommodating multiple fluid lines or risers.
Separate swivel assemblies are stacked on top of each other with a swivel stack base fixed to a stationary frame anchored to the sea floor.
Prior high pressure product swivels have provided an inner housing and an outer housing which is rotatively supported on the inner housing by a bearing so that the outer housing is free to rotate about the inner housing. A toroidally shaped conduit chamber is formed between the two housings when the two housings are placed in registration with each other. An inlet from the inner housing communicates with the chamber, and an outlet in the outer housing communicates with the chamber. Upper and lower dynamic seals in the form of face seals or radial seals are placed in grooves or gaps between axially opposed or radially opposed surfaces of the inner and outer housings to prevent fluid from leaking past the two facing surfaces while the high pressure fluid is present in the chamber.
When high pressure is present in the inlet and passes through the toroidal chamber and out the outlet, the pressure in the chamber acts to separate the inner housing and the outer housing from each other. In other words, the inner housing is forced to contract radially inward as a consequence of the force generated by the fluid pressure acting on an effective area between the two dynamic seals; the outer housing is forced to expand radially outward by the force of the fluid pressure acting on an effective area between the upper and lower dynamic seals. Separation occurs between the facing surfaces as a result of high fluid pressure in the chamber. High pressure as used herein is meant to be at the level of 2,000 psi and above.
As the pressure of flowing product increases, the separation between the facing surfaces in which the seals are placed increases. Such separation, can be large enough due to the high product pressures, so as to prevent leak-free operation of the product swivel at the high pressures by seal extrusion failure.
Swivel component deformation has been the subject of much effort by prior developers. The prior art has considered the idea of adding more material to the swivel components so that deformation as a function of pressurexe2x80x94especially high pressure in the 5,000 to 10,000 psi rangexe2x80x94will resist deflection. With high pressures, however, the swivel components, i.e., the inner and outer housings, become so large and heavy that they are disadvantageous from weight, cost, handling and size standpoints and without necessarily achieving the desired gap control.
The prior art has disclosed swivels which use exterior pressure sources to apply balancing or xe2x80x9cbufferxe2x80x9d fluid pressure at the dynamic seal interface. Examples of such xe2x80x9cactivexe2x80x9d pressure compensation for dynamic seal gap control are shown in U.S. Pat. No. 4,602,806 to Saliger; U.S. Pat. No. 4,669,758 to Feller et al., U.S. Pat. No. 5,411,298 to Pollack; U.S. Pat. No. 6,053,787 to Erstad et al., and U.S. Pat. No. 4,662,657 to Harvey et al. All of these patents disclose separate anti-extrusion rings above and below the annular fluid manifold in combination with active pressure compensation.
U.S. Pat. No. 4,555,118 to Salinger discloses at FIG. 4 a free floating anti-extrusion ring placed above and below a toroidal passage between inner and outer rings. The free floating antiextrusion ring is initially displaced (i.e., at zero pressure) from the inner joint ring by a small seal extrusion gap. In operation, the internal pressure of the pressurized fluid in the toroidal passage is transmitted to the outer side of the anti-extrusion ring such that the pressure differential across the seal presses the anti-extrusion ring against the outer surface of the inner ring. In other words, the seal extrusion gap width varies as a function of internal pressure. Metal to metal contact of the anti-extrusion ring with the annular surface of the inner ring can cause friction and scoring problems during operation.
U.S. Pat. No. 4,819,966 to Gibb at FIGS. 2, 3 and 4 shows an annular ring having an annular groove which registers with the inlet of an inner housing. An annular chamber is formed outwardly in the annular ring such that upper and lower lips are created in the annular ring which faces the exterior surface of the inner housing. The lips carry dynamic seals and are forced into sealing engagement about the cylindrical surface of the inner housing above and below the inlet when pressure is in the chamber. A constant seal gap is maintained as a function of pressure by proper shaping of the chamber and the ring and the lip. A lubricating system may also be provided for injecting a controlled fluid.
Another problem inherent in high pressure production swivels is that at extremely high pressures, e.g., 5000 psi and above, a fixed connection of the inlet spool to the inner housing can cause pipe loads and a seal stab connection can cause forces to be applied to the inner housing as a result of the pressure acting at the connection. For example, U.S. Pat. No. 6,053,787 at FIG. 2B shows a spool connected to an inner housing, and although sealed with respect to the inner housing, high pressure in the spool causes forces to be transmitted to the housing as a result of the pressure. U.S. Pat. No. 4,662,657 shows at FIG. 1 a pressure balanced connection of a spool at a swivel stack base. Such connection acts as an expansion joint in that the connection is pressure balanced and does not transmit force. This allows the spool to grow freely when heated, thus eliminating thermal loading due to pipe expansion. The ""657 patent shows a fixed connection to the inner housing of the swivel to which it is connected in the swivel stack.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a fluid swivel arrangement that is capable of flowing high pressure product through it without danger of product leaking past dynamic seal grooves formed between the inner and outer housings.
Another object of the invention is to provide a swivel arrangement with high pressure fluid in the conduit chamber, such that radial expansion of the outer housing and radial contraction of the inner housing has little or no effect on the bearing between the inner and outer housings.
Another object of the invention is to provide a high pressure swivel such that radial expansion of the outer housing or radial contraction of the inner housing has substantially no effect on the extrusion gap of dynamic seals between the inner and outer housings.
Another object of the invention is to provide a sealed fluid joint for a fluid swivel in which a middle housing ring is positioned between a conventional inner housing and outer housing ring with dynamic product seals between the inner housing and the middle housing ring and static product seals between the middle and outer housing ring.
Another object of the invention is to provide a sealed fluid joint with an internal design that minimizes dynamic extrusion gap growth as a function of internal fluid pressure by transferring component deformation from the dynamic seal gap to the static seal gap while taking advantage of the fact that static product seals can tolerate larger extrusion gaps without failure.
Another object of the invention is to provide a sealed fluid joint with an inner housing and a coaxial middle and outer housing rings which provide two concentric toroidal chambers, one formed between the inner housing ring and the middle housing ring, the second between the middle housing ring and the outer housing ring where both toroidal chambers are fluidly in communication with each other through holes in the middle housing ring thereby allowing the pressurized product fluid to flow from the inlet to the outlet.
Another object of the invention is to provide a sealed fluid joint with an inner housing and coaxial middle and outer housing rings with dynamic product seals placed between the inner housing and the middle housing ring and static product seals placed between the middle and outer housing rings with the dynamic extrusion gap width being controlled as a function of pressure acting on the difference of effective areas defined by the static seals and the dynamic seals, the shape of a toroidal chamber between the inner housing and the middle housing ring and the shape of a toroidal chamber between the middle and outer housing rings.
Another object of the invention is to provide a sealed fluid joint with an inner housing and coaxial middle and outer housing rings with dynamic seals placed in glands between the inner housing and middle housing rings, where passive control over the dynamic seal extrusion gap is obtained from the internal fluid pressure causing the middle housing ring to contract radially at the same rate as a function of pressure as does the inner housing.
Another object of the invention is to provide a sealed fluid joint with an inner housing and middle and outer housing rings where the middle and outer housing rings are arranged and designed to rotate together but can move radially away from each other as a function of increasing internal fluid pressure.
Another object of the invention is to provide a swivel stack arrangement with a swivel stack base and swivel where a floating spool provides a pressure balanced connection at the inner housing and at the swivel stack base.
The objects identified above, as well as other advantages and features of the invention, are incorporated in a sealed fluid joint for a rotatable fluid swivel in which a pressure balanced middle housing ring is mounted between an inner housing and outer housing ring. Two bearing plates bolted on the top and bottom of the middle housing capture and support the outer housing ring around the middle housing ring. The sub-assembly of the bearing plates, middle housing ring and outer housing ring is free to rotate around the inner housing.
Pressure balance is achieved by providing an inner annulus chamber or cavity between the inner housing and middle housing ring and an outer annulus chamber or cavity between the middle and outer housing rings. Holes or passages through the middle housing ring fluidly connect the inner and outer chambers. Dynamic seals are placed in seal glands between the inner housing and the middle housing ring. Static seals are placed in seal glands between the middle and outer housing rings. Seals are matched in pairs and symmetrically located above and below a swivel horizontal line of symmetry. The seal glands may be radially or axially oriented. In other words, face seals may be placed in glands which are parallel with the swivel assembly horizontal line of symmetry. Radial seals may be placed in glands which are parallel with the inner housing longitudinal axis.
The sealed fluid joint arrangement transfers component deformation due to product fluid pressure from the dynamic seal interface to the static seal interface by exposing fluid product pressure to a smaller effective area at the dynamic seals on the inner side of the middle housing ring than an effective area at the static seals on the outer side of the middle housing ring. The counter forces generated by the product fluid pressure over two different effective areas on the middle housing ring deforms the middle housing ring radially in a predetermined direction and amount as a function of increasing pressure. Control of radial deformation of the middle housing ring is passive as it relates to a geometrical arrangement of dynamic and static seals on both sides of the middle housing ring and is proportional to the product fluid pressure.
The objective of the arrangement is to maintain the dynamic extrusion gap at its initial or zero pressure width even at high internal fluid pressures. Because dynamic seals are more likely to fail than are static seals when subject to pressure, maintaining a small dynamic extrusion gap minimizes the likelihood of product leaking past the dynamic seals. Maintaining the dynamic extrusion gap at its initial width using this arrangement also eliminates the effect of radial deformation on the bearing performance, because bearing plates are provided which are bolted to the middle housing ring and subject to the same effect.
The transfer of component deformation from dynamic seal extrusion gaps to static seal extrusion gaps is provided with a coupling arrangement between the outer housing ring and the middle housing ring so that the outer and middle housing rings are capable of rotating together about the inner housing and with the outer and middle housings rings being free to deflect radially with respect to each other as a function of internal fluid pressure in the joint. The coupling insures that the outer housing ring expands concentrically with respect to the middle housing ring even when external side loads are applied.
According to another aspect of the invention, a swivel stack is provided with a fixed swivel stack base and one or more swivels having their inner housings each fixed to each other and to the fixed stack base. Each swivel has an inlet at its inner housing connected to an inlet of the stack base by means of a floating spool with a pressure balanced connection at both the swivel and at the base. Such pressure balancing of the spool at the base and the swivel inner housing prevents high pressure fluid induced forces and thermal induced forces from being transferred from the spool to the swivel as would arise with a simple stab seal connection. Providing a floating spool with a pressure balance connection results in a coupling where substantially no pipe loads are transferred to the swivel as would occur with a rigid pipe connection. Furthermore, providing a floating spool minimizes the size of the inner housing, because it has a small profile when compared to bolted flange. A small spool size minimizes the total weight of the swivel stack. The floating spool also reduces the loads on the swivel stack base and the size of the swivel itself.
According to another aspect of the invention, a swivel stack is provided with a connector positioned coaxially with the inner housings of upper and lower swivels. The connector is secured between a bottom surface of an upper swivel and a top surface of a lower swivel. Each swivel has upper and lower bearing plates which are positioned respectively adjacent upper and lower surfaces of the inner housing and a coaxial middle housing of each swivel. The bearing plates, secured to the middle housing ring, have an internal radius as measured from the central longitudinal axis. The connector has an external radius, as measured from the central longitudinal axis which is less than the internal radius of the lower and upper bearing plates.